In The Merchant of Venice, the ongoing animosity between Shylock and Antonio intensifies. Shylock is using this opportunity to air his grievances as now Antonio wants something from him. Shylock is surprised that someone who calls him "misbeliever, cut-throat dog" (I.iii.106) is now asking for help from someone he holds so contemptuously.
Antonio's response is to admit to his feelings - "I am as like to call thee so again"- and to spit at and mock ("spurn") him. Antonio shows a lack of respect. If Shylock is going to agree to lend him the money, Antonio does not want him to think that this makes it a loan between friends . When he says "a breed for barren metal" Antonio is suggesting something unnatural - "barren metal" - and he is insulting Shylock's "breed," revealing his prejudice. It is unnatural for them to be friends.
By lending money to his "enemy," according to Antonio, Shylock will be able "if he break," in other words, if he fails to repay, to "exact the penalty" and call in the debt with "better face" - without feeling guilty.